Sm14, belonging to the family of Fatty Acid Binding Proteins (FABPs), is a cross reactive antigen showing a high level of protection against schistosomiasis and fasciolosis.
Pathogens are infectious organisms, such as bacteria, virus, protozoa, helminths, or any parasite which causes infectious diseases to the host generally by expressing specific antigens which are recognized by host immune systems as foreign and become the target of an immunological response to eliminate the infectious pathogen.
Typically, there are specific sites on antigens, the binding epitopes or just epitopes, which bind to a complementary portion of a cellular protein, i.e., the receptor site. Thus, pathogen antigens often bind to cellular receptors on a host's cell as part of the process of infection of the host by the pathogen. Similar complementarity exists between host antibodies raised against an antigen and the antigenic determinants of the antigen itself. These regions of the antigenic molecule, however, may be different from those important for host cell invasion. In order to immunize the host and reduce the effectiveness of the pathogen to mount a challenge to the host, a number of vaccination strategies have been devised.
Up to recently, as described in Institute of Medicine, “Vaccine supply and innovation”, Washington, D.C.: National Academy Press (1985), several strategies have been employed to develop safe and effective vaccines consisting of live attenuated pathogens, killed pathogens, components of pathogens, or modified toxins (toxoids).
Vaccines against several pathogenic virus, bacteria, and protozoa, such as small pox, yellow fever, measles, diphtheria and malaria are available. Concerning pathogenic helminths which are parasitic worms and cause human and veterinary diseases, such as schistosomiasis and fasciolosis, at the moment, no vaccines are currently used in prevention and control programmes. These diseases are not directly transmittable from one person (or animal) to another and the helminth requires an intermediate host and environmental conditions to complete its complex life cycle. There is still a great gap in the knowledge of the variables influencing the dynamics of transmission of these diseases in connection with vaccines and vaccination protocol design. In other words, and based on the current knowledge of epidemiological parameters which modulate and influence vaccination efficacy against these diseases, it can be asserted that neither the preferential individual levels of protection required by a vaccine, nor the number of individuals to be vaccinated and/or protected among a given population have yet been established.
Nowadays, the use of vaccines composed of pathogen components or attenuated parasites for human immunization is considered impractical and potentially dangerous. The worry in using such complex and undefined mixtures comes from the fact that the majority of components stimulate non-functional immune responses and some components can even be detrimental to vaccinated subjects, when toxic products of lipid peroxidation can be generated by immune attack against other parasite antigens, particularly surface antigens.
These considerations have led researchers to seek alternative methods for effective immunization and a great deal of effort has been made to purify natural proteins from natural sources or synthetically produce them by chemical means or alternatively by using recombinant DNA technology.
Attempts to vaccinate model animals against schistosomiasis with homogenates led researchers to find a saline extract (SE) which presented good results in conferring protection against diseases caused by Schistosoma infections in humans.
Protective immunity against schistosomes, was reported on the use of a “cocktail” of schistosome components (called SE) released early during the incubation of live and freshly perfused S. mansoni adult worm in phosphate buffered saline (PBS). Focusing on attempts to achieve protection against cercarial infection by vaccination, an experimental model was designed, in two different outbred animal hosts, the SW mouse and NZ rabbit, known to be fully susceptible and partially resistant to S. mansoni infection respectively.
Studies on the induced immune response in vaccinated animals aiming at the identification of the functionally relevant SE protective components, the site and mechanism of parasite death as well as markers of protection, have been the focus of our efforts in recent years. Less information on the molecular composition of SE, as well as on the identification and isolation of its protective components has been available until recently. (see: Tendler, M. and Scapin, M. (1979). “The presence of Schistosoma mansoni antigens in solutions used for storing adult worms”. Rev. Inst. Med. Trop. 21(6): 293-296; Tendler, M et al. (1982). “Immunogenic and protective activity of an extract of Schistosoma mansoni”. Mem. Inst. Oswaldo Cruz. 77(3): 275-283).
The U.S. Pat. No. 4,396,600 issued on Aug. 2, 1983 in the name of Luigi Messineo & Mauro Scarpin described an extract of adult Schistosome mansoni worms obtained by incubation in 0.15M sodium chloride-sodium phosphate buffer pH 5.8. The extract contains protein, carboxydrates, and nucleic acid and or by-products of the latter component and resolves into four major fractions (I-IV) by gel chromatography in G-100 and G-200 Sephadex columns. Immunodiffusion tests with rabbit anti-total extract serum reveal three precipitation lines corresponding to fractions I and II and one with III or IV. Rabbits immunized with this total extract are found to be totally or partially (at least 77%) resistant to a challenge infection. The saline extract antigenic material is an effective vaccine for the treatment and immunization of schistosomiasis and other schistosome infections.
Another published study is “A 14-KDa Schistosoma mansoni Polypeptide is Homologous to a gene family of fatty Acid Binding Proteins—The Journal of Biological Chemistry—vol. 266, No. 13, Issue of May 5, pp. 8447-8454, 1991; D. Moser, M. Tendler, G. Griffiths, and Mo-Quen Klinkert”. This study describes the sequencing of the gene and the demonstration of the functional activity of Sm-14 as a protein which binds lipids.
Thus, schistosome antigens present in SE and other related helminth antigens have been cloned, sequenced, characterized, and the corresponding recombinant proteins prepared. Examples are: Sm14 (U.S. Pat. No. 5,730,984 granted to Fundação Oswaldo Cruz on 24 Mar. 1998; Fh-15 (Perez et al. (1992). “Fasciola hepatica: Molecular cloning. Nucleotide sequence and expression of gene encoding a polypeptide homologous to a Schistosoma mansoni Fatty Acid-Binding Protein”. J. Exp. Parasitol. 74(4): 400-407.
However, vaccines which are based on the use of proteins belonging to the pathogen, be they altered or not, are not always easily obtainable. Difficulties in the extraction, purification, quantitative analysis and modification of such proteins are common problems with this type of vaccine. Solutions exist for some such cases but these may result in an additional onus to the protein production process which goes against the general principle that a vaccine should be of relatively low cost and should be globally accessible.
As an alternative, although not without its own deficiencies, is the use of synthetic peptides as vaccines.
There were attempts to combine epitopic portions of more than one antigen to raise their immunological properties. An example of this approach is described in the U.S. Pat. No. 5,219,566 granted to The John Hopkins University on 15 Jun. 1993 and refers to the construction of polypeptides based on the identification of epitopic regions which are common to two S. mansoni proteins. The polypeptides have epitopes which are shared by the 200 and 38 kDa proteins of S. mansoni and are able to bind to protein epitopes but not glycan epitopes expressed on the surface of live schistosomula of S. mansoni. The epitope (or epitopes) on the 38 kDa protein are exposed to the surface of the schistosomula while the epitope on the 200 kDa protein is apparently not exposed to the surface of schistosomula. A fusion protein having portions of any bacterial protein which is well expressed, particularly using portions of the amino terminal end of the enzyme beta-galactosidase, is included in the invention. It is mentioned that the particular subset of adult worm antigens was selected based on its enhanced reactivity with sera of vaccinated as compared to chronically infected mice.
Although many antigens from helminths are available and have been studied in connection with their protective potential only six Schistosoma mansoni antigens were selected by the WHO (World Health Organization) as vaccine candidates against diseases caused by schistosomes (see Progress Report 1975-94, Highlights 1993-94—20 Years of Progress, Tropical Disease Research WHO, Geneva, 1995). The selected antigens are: GST-28 kDa (also known as Sh28-GST)—a Glutathione S-Transferase, which is located in the schistosomula or adult worm parenchyma and in the adult worm backbone; Paramyosin-97 kDa—a muscle protein from adult worms or schistosomula; Sm23-23 kDa—a membrane protein from adult worms; IrV5-62 kDa—a protein which is homologous to myosin and is present in all parasite stages; TPI-28 kDa—a Triose Phosphate Isomerase and rSm14-14 kDa—from adult worms and belonging to the Fatty Acid-Binding Protein family.
Of these six vaccine candidates against S. mansoni initially selected by the WHO, four have been subsequently endorsed for scale-up to GMP grade antigen production and phase I/II clinical trials in humans. Two of these, Sh28-GST and Sm14 are closest to reaching this reality with GST already in phase II clinical trials for S. haematobium in Senegal and Sm14 in the final stages of scale-up. Furthermore, Sm14 is the only vaccine candidate to have been shown to afford significant immune protection against two relevant helminthic diseases of human and veterinary importance, namely Schistosomiasis and Fascioliasis. 
Sm14 is thus a unique opportunity for attacking both the second most prevalent parasitic disease in humans—Schistosomiasis—and the most important helminth infection of cattle—Fascioliasis—and therefore represents an attractive strategy for helminth vaccine development.
However, while some success has been achieved, these molecules are quite large.
A method currently under intensive investigation is the use of synthetic peptides corresponding to segments of the proteins from the pathogenic organism against which an immune response is directed. When these peptides are capable of eliciting a neutralizing immune response they appear to be ideal immunogens. They elicit a specific response and typically do not lead to deleterious effects on the host. However, it can be difficult to predict which peptide fragments will be immunogenic and lead to the development of a neutralizing response. It could be desirable to develop immunogens that elicit a response to specific neutralizing epitopes without causing responses to extraneous epitopes that could “dilute” the specific response or lead to harmful immune complex formation, including autoimmune reactions.
Such a method is accomplished by the identification of specific and discrete portions of proteins involved in the protein-protein interactions relevant to the immune response and the construction of biologically active peptides based upon the amino acid sequences identified.
Protein binding or protein-protein interactions can be broadly defined as an example of molecular recognition in which the surfaces of two macromolecules (proteins) or a peptide and a protein present discrete surface interactions involving chemical and shape complementarity. Such discrete interactions arise when residues of one protein (or peptide) are located spacially close to residues of another protein and attractive forces between the residues such as Van der Waals forces, salt bridges, hydrogen bonds, and hydrophobic interactions exist. The three-dimensional disposition of specific kinds of residues allows attachment to occur as a consequence of a large number of the above-mentioned weak interactions which together lead to a significant binding energy between the different proteins.
The hypervariable loops that occur in the complementarity determining regions of antibodies for example, on interacting with antigen epitopes may employ the wide range of chemical interactions described above. The binding surface or cavity on the antibody (paratope) is formed by the spacial distribution of the residues which comprise the variable domain of the antibody's light and heavy chains and particularly the hypervariable regions responsible for antigen complimentarity. Good fit of the antigen's epitope into the antibody's paratope depends on the shape and chemical nature of both components. The affinity of a given antibody for its antigen depends on the sum of the attractive and repulsive forces between epitope and paratope. However, since an antibody possesses two paratopes and given that many antigens are multivalent in nature, the overall antibody avidity will depend on the total number of paratopes and epitopes involved in the interaction.
A wide variety of topographies are observed for antibody combining sites. They may be relatively flat surfaces (common in the case of protein antigens), grooves (as is often the case for peptides) or cavities (in the case of small molecule haptens). Often exposed, flexible and highly protruding parts of a protein antigen (often corresponding to surface loops on the structure), are the immunodominant epitopes and there is evidence to suggest that there is flexibility in both the antigen and antibody which is necessary for an optimal ‘induced’ fit on complex formation.
Similarly for other types of molecular recognition important in the immune response, individual structural elements of the proteins involved are fundamental for the specificity. This is true for example in HLA interactions with processed peptides and in the interaction of the T-cell receptor with such HLA-peptide complexes.
By identifying the specific and discrete portions which confer antigenic properties to a particular protein, biologically active peptides can be constructed to mimic pathogen antigens and act on mammalian cells by binding to the receptor sites of those cells to alter or affect their function or behavior, or to prevent or alter the effect which pathogen antigens would otherwise have upon those cells. Such mimicking molecules would be useful as agents to affect the cells in the same manner as the natural protein. Alternatively such peptides may bind to soluble antibody.
As such, active peptides derived in this manner may elicit either T-lymphocyte and/or B-lymphocyte immune responses. Accordingly, the document WO91/09621 discloses a peptide fragment bearing amino acid sequences of the 28 kDa Schistosoma mansoni antigen which shares at least one epitope which induces a T-lymphocytes specific response and at least one epitope which induces a B-lymphocytes specific response. The peptide fragment described in that patent application corresponds to 2n-fold the amino acids 115-131 of the 28 kDa Schistosoma mansoni protein. It is mentioned that the advantage in using such a peptide fragment is to induce both humoral and cellular responses while the original protein (28 kDa of S. mansoni) does not induce almost any humoral response. It is also mentioned that the peptide fragment confers a protection of about 40-50% in animal models (rats).
EP 251 933 proposes a process for isolating a peptide fragment bearing at least one epitope of the 28 kDa of S. mansoni by subjecting, under controlled proteolytic conditions, the 28 kDa polypeptide of S. mansoni to the action of the protease V8. The applicant mentions that the preferred peptide fragments are those of 8 kDa and of 6 kDa which bear the anti-Schistosoma antigenic activity shown by the 28 kDa protein. The amino acid sequences of the peptide fragments were not disclosed in the patent application.
Synthetic vaccines which comprise a peptide fragment of sufficient size are considered to be of critical importance in providing the active portion or portions of the entire antigen which can be recognized by the immune system and evoke formation of the corresponding antibodies. Biologically active peptides can be constructed which function as the epitope or mimic a biologically active protein. Alternatively, biologically active peptides can be constructed which interact with receptors and thereby block the binding of a pathogen antigen or biologically active protein to a receptor.
U.S. Pat. No. 5,019,383 describes a synthetic vaccine comprising a peptide residue coupled to one or more alkyl or alkenyl groups of at least 12 carbon atoms in length or other lipophylic substance. It is described that the peptide residue contains a sequence of 6 amino acids corresponding to the sequence of such amino acids in a protein antigen or allergen where the greatest local average hydrophilicity of the antigen or allergen is found. Moreover, it is mentioned that the alkyl or alkenyl groups are the carrier on which the peptide residue is disposed, said carrier being of critical importance in providing the active portion of the synthetic peptide chain with sufficient size so that the entire synthetic antigen or synthetic allergen can be recognized by the immune system and evoke formation of the corresponding antibodies. It is also described that the synthetic peptide residue has a chain length of minimally six amino acids, preferably twelve amino acids and can contain an infinitely long chain of amino acids or their components, which can be characterized by the presence of other epitopes of the same or different antigen or allergen.
Another example of an active peptides approach is disclosed in the patent application WO93/23542 which refers to nucleic acid molecules containing nucleotide sequences encoding helminth aminopeptidase enzymes, and antigenic fragments and functionally-equivalent variants thereof, their use in the preparation of vaccines for use against helminth parasites, and synthetic polypeptides encoded by them. The invention of WO93/23542 is based upon the role of mammalian integral membrane aminopeptidases in cleaving the small peptides which are the final products of digestion.
In short, the identification of epitopic regions of pathogen antigens or biologically active proteins can be used in the construction of biologically active compounds which comprise equivalent or shared amino acid sequences. Furthermore, biologically active compounds, such as peptides, can be modelled based upon amino acid sequences of biologically active proteins whose epitopic regions are known.
However, peptides are basically fragments of polypeptide chain of limited size, which may be sometimes modified with respect to the original sequence of amino acids as found in the parent protein from which the fragment is derived. This presents a series of problems. Factors such as peptide solubility, degradation, aggregation, conformational stability, among others, are relevant given the final objective intended for the peptide.
As discussed above, Sm14 is a protein of particular interest. It is a cross reactive antigen which confers protection against both schistosomiasis and fasciolosis (see Tendler, M. et al. (1996). “A Schistosoma mansoni fatty acid binding protein, Sm14, is the potential basis of a dual-purpose anti-helminth vaccine”. Proc. Natl. Acad. Sci. 93: 269-273 and U.S. Pat. No. 5,730,984). The data presented in these publications show the effectiveness of Sm14 in conferring high levels of protection against helminth infections.
Thus, the antigen Sm14 is an especially suitable active protein which can be used to model biologically active peptides based upon the determination of its cross-reactive epitopes.